The premise of the measurement comparison method is that the induction melting furnace maintenance personnel should understand or actually measure the correct voltage value and waveform of the key parts of the printed circuit board and the faulty parts, so that the comparison analysis can be performed, and the data should be ready at any time. Record and accumulate as information. Common instrumentation measurement methods are as follows.

    1) Voltage measurement method

    A voltage measurement method is a detection method for determining a fault by measuring an operating voltage of an electronic circuit or a component and comparing it with a normal value. Voltage measurement is the most basic and most common method of all detection methods. The voltages that are often tested are the power supply voltages at each stage, the voltages at the poles of the transistors, and the voltage at each pin of the IC. In general, the measured voltage results are an important basis for reflecting the normal operation of the induction melting furnace. Where the voltage deviates from the normal value, it is often

The location where the barrier is located. Voltage measurement methods can be divided into AC voltage detection and DC voltage detection.

(l) AC voltage detection. In the induction melting furnace circuit, because the AC circuit is less, the circuit is relatively uncomplicated, and the measurement is relatively simple. Generally, the AC voltage of the multimeter can be used to measure from the input end of the induction melting furnace. If it is normal, the AC terminal of the rectifier module is detected again. Whether the voltage is normal, to determine the front end of the power failure. For the measurement of the intermediate frequency voltage at the output of the induction melting furnace, the water-cooled cable at the output of the induction melting furnace can be removed and then measured to determine the fault location.

    (2) DC voltage detection. For the detection of DC voltage, first detect the output of the rectifier circuit of the main circuit, the input of the inverter circuit, and then detect the input and output of the control power supply circuit and the voltage regulator circuit, according to the measured voltage of the input terminal and the output terminal. Determine which part of the circuit or a component is faulty. When measuring the unit circuit voltage, first measure the power circuit of the unit circuit. If the voltage is too high or too low, the unit circuit is faulty. DC voltage

When detecting the working voltage of each leg of the integrated circuit, the data provided by the maintenance data should be compared with the measured value to determine the quality of the integrated circuit.

    Generally, the AC voltage and DC voltage can be used to directly use the multimeter, but pay attention to the range and gear selection of the multimeter. The voltage measurement is a parallel measurement, and the concentration must be concentrated during the measurement process to prevent the multimeter's test leads from shorting the two solder joints.

    2) Current measurement method

    The current measurement method is an inspection method for judging the fault of the induction melting furnace by detecting the operating current of the transistor, the integrated circuit, the current of each unit circuit, and the load current of the control power source. When the electronic circuit is detected by the current measurement method, the cause of heat generation of the transistor and the heating of the electrical component can be quickly found, and it is also a common means for detecting the working state of the integrated circuit.

  When using current measurement, it is often necessary to disconnect the circuit. It is difficult to implement the multimeter in series with the multimeter. For this reason, the current measurement method is divided into direct measurement method and indirect measurement method.

    The indirect measurement method actually uses the measured voltage to convert the current or uses a special method to estimate the current. For example, when measuring a certain pole current of a transistor, the current value can be converted by measuring the voltage drop of the series resistor on the collector or emitter. The advantage of this method is that it is not necessary to make a measuring port on a printed circuit board. In addition, some boards have temperature-preserving resistors on critical circuits. By measuring the voltage drop across such resistors, and applying Ohm's law, the magnitude of the load current in each path can be estimated. If a circuit temperature fuse is blown, the current can be directly measured by the current of the multimeter to determine the cause of the fault.

   When the fuse of the induction melting furnace fuse is blown or the partial circuit is short-circuited, the current measurement method has obvious detection effect. The current measurement is a series measurement, and the voltage measurement is a parallel measurement. In actual operation, the voltage measurement method is often used first, and if necessary, the current measurement method is performed.

    3) Resistance measurement

    The resistance measurement method can be divided into a step measurement method and a segment measurement method. Resistance measurement is a method of judging a fault by measuring the resistance of a component to ground or its own. It is effective for overhauling open circuits, short circuit faults, and identifying faulty components. By measuring the resistance values of resistors, capacitors, inductors, coils, transistors, and integrated circuits, the specific location of the fault can be determined.

    Resistance measurement is one of the most basic methods for troubleshooting. In general, resistance measurement has two methods: “on-line” measurement resistance and “offline” measurement resistance. When measuring the resistance "online", since the component to be tested is connected to the whole circuit, the resistance value measured by the multimeter is affected by other parallel branches, and should be considered when analyzing the test results to avoid misjudgment. The measured resistance value will be equal to or smaller than the actual marked resistance of the component. It is impossible to have a larger than the actual labeled resistance. If so, the measured component is faulty.

    When measuring the resistance “offline”, it is necessary to take off one end of the component to be tested or remove the entire component from the printed circuit board, and then measure the resistance with a multimeter. This method is cumbersome to operate, but the measurement result is accurate and reliable. .

    When using the resistance measurement method, it is generally first to test the resistance of the line resistance, measure the resistance value of each component, and exchange the red and black test leads of the multimeter, and then test the resistance value. This will eliminate the interference of the external circuit network on the measurement results. It is necessary to analyze the measured resistance values twice, and further test the components that are suspected to be desoldable. Online testing must be performed in the event of a power outage, otherwise the measured results are not accurate. Will also damage, damage the multimeter, in the detection of some low electricity

When using an integrated circuit such as 5V, 3V, do not use the R x10k file of the multimeter to avoid damage to the integrated circuit.

The measurement method should pay attention to the following points in practical applications.

(1) Pay attention to the common grounding in the test. In order to make the inspection normal, the instrumentation and the inductive melting furnace to be repaired should have a common grounding point.

(2) Pay attention to the phenomenon of high voltage "string point, string line". Inductive melting furnaces that are faulty often have insulation breakdown, which causes high-voltage stringing and stringing, which will endanger personal safety and damage measuring instruments, and affect measurement data. This should be noted.

(3) Obey the maintenance procedures of “power off before measurement, reconnect after disconnection”. Especially when measuring high voltage, the power should be cut off first. To prevent the charge stored in the bulk capacitor from being shocked, fully discharge it before connecting the test leads. After the test line is connected to the high voltage point, the power is turned on to ensure personal safety.

(4) The test line should have good insulation.

(5) Before the test, the Australian instrument should have a full understanding of the detection instrument and the principle of the circuit being tested.

(6) It is necessary to develop the habit of one-handed measurement to prevent the hands from touching the charged body at the same time, which jeopardizes personal safety and damages the measuring instrument.